The Chicago cohort of the Multicenter AIDS Cohort Study (MACS) was initially recruited between April 1984 and March 1985. Eleven hundred and two (1102) homosexual/bisexual men volunteered for the study during the enrollment period. To increase participation by African-Americans enrollment was reopened in 1987; 249 new volunteers were recruited. The objective of the MACS is to define the natural history of infection due to the human immunodeficiency virus, type-1 (HIV-1). Specific aims include: maintenance of the cohort, continuation of participation by members of the cohort in scheduled semi-annual clinic visits (emphasis is placed upon follow-up of seropositive individuals, specifically those with low CD4+ lymphocyte count and symptomatic HIV-1 infection), definition of clinical progression and impact of therapy, investigation of immunopathogenesis of HIV-1 infection, investigation of virologic determinants of progression, evaluation of neuropsychologic consequences of HIV-1 infection, study of development of malignancies in members of the cohort and participation in the investigation of health services utilization by HIV-1 infected volunteers and those at high risk of infection. To accomplish these aims, the Chicago MACS maintains: clinics to accommodate participant visits, accurate records of clinical data, continued specimen collection, properly equipped laboratories capable of handling and storing biologic specimens, and coded data files linked to personal identifiers. This work is carried out in collaboration with investigators at the other MACS sites (John Hopkins University, University of Pittsburgh, and University of California, Los Angeles), the Center for Analysis for the MACS (CAMACS) and the Division of AIDS-NIAID.